An example of related pressure vessel used for manufacturing single crystals is shown in FIG. 9.
Seed crystals 10, raw material 11, a convection control plate 9 and solvent are accommodated in a cylindrical pressure vessel 30. One end of the pressure vessel body 30 is open. Through a ring-shaped self-contraction gasket 32 arranged in a periphery of the opening, a cover 33 is fixed by a box nut or a clamp 34, so that the opening can be closed. On an inner face of the cover 33, a lining cover 35 is provided. In the periphery of the pressure vessel body 30, a heater 4 is arranged so that the pressure vessel body 30 can be heated.
When the hydrothermal synthesis method is taken up as an example, single crystals to be synthesized are artificial quartz (silicon dioxide) or zinc oxide. A strong base aqueous solution is used as a solvent. In a case of ammono-thermal synthesis method, as proposed in Patent Document 1, single crystals to be synthesized are made of gallium nitride, and liquefied ammonia is used as a solvent.
Single crystals to be manufactured are used for various optical and electronic elements. A main factor which has influence on a quality of single crystals is contamination of foreign objects caused at the time of growth of crystals. Since a solvent made of an alkali aqueous solution is corrosive, the pressure vessel body 30 and the cover 33 are corroded and ions of iron and nickel are eluted. These metallic ions are changed into chemical compounds such as oxide or nitride and mixed into the crystals in the form of foreign objects. Accordingly, it is difficult to manufacture single crystals of high purity.
As a countermeasure of solving the above problems, for example, Patent Document 2 proposes the following method. As shown in FIG. 9, a cylindrical mechanical lining 31 made of alloy having high corrosion resistance property is provided tightly close to an inner face of the pressure vessel body 30 so as to cover the inner face of the pressure vessel with the mechanical lining 31. Incidentally, the method of covering the inner face of the pressure vessel with corrosion-resistant material is proposed in Patent Documents 3 and 4 in which the inside of a reaction container used for supercritical water is covered with the mechanical lining.
Except for the above method, the following method is proposed. As shown in FIG. 10, an internal cylindrical container 37 made of corrosion-resistant alloy is accommodated in the pressure vessel body 30. This technique is disclosed, for example, in Patent Documents 5, 6 and 7.
The internal cylindrical container 37 is separate from the pressure vessel body 30. At least an inner face of the internal cylindrical container 37 is made of precious metal such as Pt having high corrosion resistance property so that foreign objects can be prevented from being mixed into single crystals. When this method of using the internal cylindrical container 37 is employed, since the internal cylindrical container 37 is liable to break when a pressure difference is given between the inside and the outside of the internal cylindrical container 37, it is necessary to apply a technique of making the pressure difference, which is given between the inside and the outside of the internal cylindrical container 37, to be uniform. Accordingly, in this method, a pressure regulator 38 is attached. Patent Document 8 proposes an internal cylindrical container 37 which is made of material capable of being easily plastically deformed and sealed by means of cold welding.
Patent Document 1: JP-A-2003-277182
Patent Document 2: JP-A-2003-165794
Patent Document 3: JP-A-2001-170478
Patent Document 4: JP-A-2002-361069
Patent Document 5: JP-B-7-22692
Patent Document 6: JP-A-2003-63889
Patent Document 7: JP-A-2003-176197
Patent Document 8: US 2003/0141301 A1